With millions of square feet of office space, a highly profitable shopping center and thousands of apartments and hotel rooms, Tysons Corner is Fairfax County's biggest money tree.

But the bountiful tree, at the county's busiest crossroads, is bearing some bitter fruit.

First there is the transportation mess: About 100,000 cars must trickle through the tangled road network every day; mass transit is marginal and there is no well-organized carpooling for the thousands who work there. In typical suburban fashion, most of the office workers are isolated from the major shopping areas, so that just to go to lunch, most employes need a car.

Then there is the visual mess: Some of the prime land areas that designers once considered ideal for campus-like corporate parks have become garishly lighted outdoor warehouses for the car dealerships that line both sides of Rte. 7. The office towers, the most visible part of Tysons, look as if they were designed by a computer-programmed and very cost-conscious accountant.

Most observers agree that it is too late to reverse the hodge-podge at Tysons, but they say the county now has a chance to avoid repeating the mistakes at what has become the center of new development.

Attention now is focused on 50/66, shorthand for 1,640 acres straddling Rte. 50 and I-66 just west of Fairfax City. As western Fairfax County continues to grow, the area is destined to become the new population center of the county.

Most of 50/66 is still a clean slate, but that already is changing. A new regional shopping center, Fair Oaks, will have six department stores and more than 150 shops when it opens this summer.

On Rte. 29-211, another main road just south of Rte. 50 and I-66, the county hopes to build a new governmental seat and cultural center on 183 acres it bought last year.

Though construction of the governmental complex requires the approval of the voters, county officials already are moving forward with an architectural competition for the center -- evidence of a newly discovered concern for quality as well as quantity.

Despite the county's determination, several questions are still unanswered. The most crucial, of course, is what private developers will do with the rest of the land at 50/66: Will they conform to the high standards the county seems to be setting for itself? How will traffic flow through the area? What provisions will be made for pedestrians? What about mass transit? What about provisions for open space?

In short, will 50/66 become another Tysons?

Fearful that history could repeat itself, the county is about to declare a time-out on further planning for 50/66 until it can answer the questions that were pushed aside at Tysons Corner.

For several months, a variety of voices have been saying, "Let's not repeat Tysons Corner." At a county Planning Commission meeting last week, Michael S. Horwatt, chairman of the Governmental Center Master Plan Committee, came right to the point:

"If we are to prevent another Tysons Corner," he said, "a sprawling kind of development that is without beauty and higher aspirations, we have got to come up with some innovative planning."

The committee, which was appointed by the Board of Supervisors last year to suggest what elements should be included in the governmental center and the nearby area, called for a timeout to take stock of the 50/66 situation. And that's what the Planning Commission recommended to the Board of Supervisors, which is likely to endorse the action next month.

In what amounts to a frank acknowledgment that the county doesn't have the tools, or maybe even the will, to avoid another planning disaster, county officials want to hold a one-day "brainstorming" workshop where officials, concerned residents, developers and outside experts can rub ideas together and perhaps spark some fresh and usable proposals.

What's wrong and what needs to be done were laid out in a recent, remarkably candid memo by David A. Edwards, executive director of the Fairfax County Economic Development Authority. The authority is usually a tireless booster of business, and has often sung paeans to Tysons Center. But not this time.

"It has been very troubling to me," Edwards wrote, "that in many respects we seem doomed to make the same planning mistakes in the 50/66 area that we made at Tysons. Failure to benefit from the experience of 15 years of watching Tysons Corner develop would be inexcusable."

In his memo, Edwards went to the heart of the problem, explaining why, despite all sorts of planning rituals, a Tysons Corner can still happen.

"We seem again to be locked into the conventional two-dimensional planning process for this area (50/66)," Edwards wrote. "While this may have validity in other, less complex areas in the county, the traditional planning and zone process is not adequate to meet the demanding needs of a complex area such as (50/66)."

Edwards then described what he thought was needed: "A three-dimensional, urban-design approach to planning which allows the various uses to better . . . function together in a truly urban and urbane setting."

Edwards stressed "that we are talking about urban development . . . and no longer suburban development. . . . While there is strong reluctance today to think in terms of dozens of tall buildings and thousands of apartments at this location, this type of development will take place here over the next 20 years. Dramatic change will take place, which cannot now be fully anticipated or realistically prevented even if we wanted to."

With the planning moratorium extending until 1981 -- during which the county hopes developers will halt construction -- the principals in the development of 50/66 will have plenty of time to chew over what Edwards had to say.

But those principals will have a lot to reconcile. For instance, there are residents, such as those of Random Hills, who want to preserve their small neighborhood despite what Edwards says is inevitable. There are landowners who have been sitting tight for 20 years waiting for the right time to sell, who say that only intense development can give them the "highest and best use of their land."

And there is the county planning office. Its harried professionals, being careful to keep their eyes on the political weathervane, are likely to try to please everybody, and consequently nobody. Already, the planning staff is finding itself in that squeeze -- at last week's Planning Commission meeting the staff had to do a humiliating flip-flop and withdraw its recommendation that master plan revisions were needed immediately.

What the 50/66 area needs are planners and public officials who speak as plainly as Edwards. Let them agree or let them disagree, but they should speak as plainly.

In Fairfax County, that would be innovative planning.